This invention relates generally to cardic pacers having systems for automatically determining the minimum pacing pulse energy required to reliably stimulate a patient's heart, and more particularly to an improved system for use in such pacers wherein the detection of induced cardiac contractions is accomplished with reduced battery current drain.
The minimization of battery current drain in cardiac pacers is an important consideration in the design and operation of such devices. This is particularly true in the case of implantable pacers wherein premature battery failure necessitates invasive battery replacement surgery. Various improvements have therefore been made in cardiac pacer design and operation which seek to reduce battery current drain while nevertheless maintaining a high degree of pacing reliability.
One such improvement is described in the copending application Ser. No. 738,606, of the present inventor entitled "Cardiac Pacer Incorporating Means for Periodically Determinating Capture Threshold and Adjusting Pulse Output Level Accordingly". As described therein, a cardiac pacer is equipped with a system for automatically determining the minimum pacing pulse energy required to reliably stimulate a patient's heart. Once this level, known as the patient's capture threshold, is determined, pacing pulses at a fixed increment above the threshold level are generated and applied to the heart. Thereafter, single pacing pulses are provided. Periodically, the capture threshold is re-determined and the pacing pulse energy reset in accordance therewith. This permits the pacer to operate slightly above the actual capture threshold level without sacrificing pacing reliability and results in a significant reduction in battery current drain.
In such a pacer, pacing pulses are initially generated and applied to the heart in pairs while the cardiac response waveforms produced by each pulse of pair are compared in order to detect any signal components indicative of induced contractions. Upon the passage of a predetermined period of time, paired pulses are once again produced and the capture threshold is re-determined. While the ability to operate the pacer slightly above a patient's actual capture threshold significantly reduces battery current drain, the need to periodically re-determine capture threshold in itself wastes battery power if in fact no change in capture threshold has occurred. Also, pacing reliability could be compromised if capture threshold changes significantly during the single pulse time period.
The present invention is directed to an improvement in the above-described pacer which results in greater pacing reliability and possibly a further reduction in battery drain. This is accomplished by means of an improved capture detection system which effectively verifies induced cardiac contractions through use of a single pacing pulse and which redetermines capture threshold only when necessary. By eliminating the need for unnecessary paired-pulse redeterminations of capture threshold, substantial reduction in battery current is realized while the capability of continually verifying induced contractions improves the overall reliability of the pacer.
In view of the foregoing, it is a general object of the present invention to provide a new and improved low power consumption system for detecting evoked cardiac contractions.
It is a further object of the present invention to provide an improved cardiac pacer in which battery current drain is minimized.
It is still a further object of the present invention to provide a cardiac pacer in which capture is verified through use of a single applied pacing pulse and in which capture threshold is re-determined only when necessary.